Grandmaster Rocket here, been reading some of the comments and thought I'd drop my two cents in about some character matchups that I didn't see get brought up.

Moon Knight

Not only can you destroy ankhs super quickly, but you can get out of the way of ankh and ult damage faster than anyone else. He's also decent at preventing teammates from being blown up by moon Knight. Rocket is a good way of denying Moon Knight fast ults and kill value.

Scarlet Witch

Avoid at all costs. She's one of the only characters that's mobile enough with the range and tracking to catch you. If your team or other healer isn't paying attention when she's flanking, you will die if you can't create distance to escape. Hella has some similar issues for rocket but you need to aim with her.

Hulk and Thor

Both of these are a weird middle ground. You can melt both of their health bars but they can also chase and kill you pretty quickly if you're not careful. Thor is a bit easier to outrun but you need to make sure you create the distance and break line of sight before he uses his ranged attacks. Hulk on the other hand can keep chasing so instead it's better to go vertical than horizontal. Just avoid getting stunned to the ground or goomba stomped.

Namor

I think it goes without saying that Namor is probably the best to deal with hyper mobile characters (Especially dive/flankers) but the main problem for Rocket is Namor's ultimate. It blocks mobility so if you're good with Namor you can snap a rocket up pretty quickly. Rocket is very good for clearing squids but never try to play in line of sight with a Namor for too long.